The present invention relates to data storage systems, and more specifically, this invention relates to recall operations in hierarchical storage management (HSM) systems.
An ever increasing amount of computer readable storage space is needed to keep pace with expanding data storage demands. Increasing data storage capacity requires improved storage management systems to backup and protect data sets, and migrate less active data sets to secondary storage to increase primary storage space. A data set may be composed of any collection or grouping of data. In certain systems, a data set may include control information used by the system to manage the data. The terms data set and file are generally equivalent and sometimes are used interchangeably. Hierarchical storage management (HSM) programs manage storage devices, such as tape libraries, to control the flow of data between primary and secondary storage facilities.
In a hierarchical storage management system, data is stored in different types of storage devices depending upon the frequency of usage of the data. For instance, a system may include multiple storage media types to store data having different usage patterns and likelihoods of access. More frequently used data may be stored on direct access storage devices (DASD) comprising high-performance rapid access storage devices, such as hard disk drives. Less frequently used data may be archived on slower and less expensive, demountable storage media, such as optical disks, magnetic tape cartridges, etc.
Two common functions initiated by host systems in hierarchical storage management systems include migration and recall. Migration typically involves the movement of data from a rapid access storage device to a slower access storage device, e.g. a tape cartridge. Conversely, a recall operation generally involves data transfer in the opposite direction. For example, when a migrated data set stored on a tape volume is requested by an application in a recall operation, the respective tape volume is mounted, the tape drive moves to the location of the data records associated with the data set, and the requested data records are read.
With regard to recall operations in HSM systems, the time taken to recall a file from the mount point of the respective tape volume is important. The time to locate a requested data record may be influenced by the length of the tape medium, the reposition velocity, the physical position of the requested data sets on the tape media, etc. Additionally, a tape drive may not know, with certainty, the actual physical position of the target data files on the tape medium, which may lead to longer than desired elapsed times.
Moreover, the time that elapses to recall a file from a lower storage tier to a higher storage tier in an HSM system may be influenced and/or longer than desired due to the difference in data rates associated with a host reading data from a tape drive versus a tape drive reading data from a tape medium. For instance, the data rate for a tape drive to read data from a tape medium is generally much faster than the data rate for a host to read data from the tape drive. As an illustration only, consider the case where the total bandwidth of a higher storage tier (e.g. comprising hard disk storage) is 2500 MB/sec, and an application running on a host system uses 2000 MB/sec. The total bandwidth for a recall operation is 2500 MB/sec−2000 MB/sec=500 MB/sec. If there are 10 tape drives, the data rate between the higher storage tier (the hard disk drives) and each tape drive is only 500 MB/sec/10=50 MB/sec, which may be significantly slower than a tape drive having a data rate of, for example, 250 MB/sec.
Accordingly, it would be beneficial to have a system, method and/or computer program product which could reduce the inefficiencies in accessing data during a recall operation in systems which employ hierarchical storage.